In recent years, development to make a liquid crystal display device thinner, lighter, larger in screen size and higher in fineness has been advancing. With this development, a request to make a protection film for liquid crystal polarizing plates thinner, wider in width, and higher in quality becomes strong. Generally, a cellulose ester film is widely used for the protection film for polarizing plates. With the trend to make a screen larger, an original film of the cellulose ester film is required to be wider in film width and longer in roll form.
Conventionally, in manufacture of an optical film by a melt casting film producing method with a so-called touch roller method, an elastic roller has been used. However, there is a problem that when the width of a original is widened, even if an elastic touch roller is used, the touch roller does not come in contact a film surface uniformly due to a deflection of the roller.
If the film thickness of an original film is made in a form of a convex shaped from both end portions to a center portion in a widthwise direction of a film (hereafter, referred to as a center portion thicker film), the problem caused by a deflection of the roller may be solved. However, there is another problem that the film with the convex becomes out of a specification as a product. Further, when a touch roller presses strongly a film at the time of casting in a film production by a touch roller method, residual strain takes place in the film. Thereafter, when the film is stretched in a stretching process, stretching unevenness takes place due to the residual strain, and layer thickness unevenness take place in the resultant film.
As a patent document in connection with a production method of an optical film which conventionally uses an elastic touch roller in order to improve such residual strain, there is the following document:    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-334842 official report